The present invention is directed to the controlled growth of nanotubes, and more particularly to the pattern-aligned growth of carbon nanotubes for use as tunable high-Q resonators.
Nanoscale structures are becoming increasingly important because they provide the basis for devices with dramatically reduced power and mass, while simultaneously having enhanced capabilities.
For example, mechanical resonators have been of significant interest because such resonators can exhibit orders of magnitude higher quality factors (Q) than electronic resonators. Such low-loss resonators are important for communications and mechanical signal processing applications. However, practical mechanical resonators have typically used bulk acoustic wave (BAW) oscillators, such as quartz crystals; or surface acoustic wave (SAW) oscillators. These bulk-crystal-based oscillators tend to be bulky and difficult to inexpensively integrate with high frequency electronic circuits. As a result, there has been a move towards Si-micromachined resonators, which can be monolithically integrated with conventional electronics. However, to date Si-based resonators have not been demonstrated at frequencies over 400 MHz, they suffer from reduced quality factors at higher frequencies, and they have limited tuning ranges. In addition, it appears that practical transduction mechanisms for the readout of Si resonators will be problematic for Si oscillators operating at or near the GHz frequency regime. Finally, data suggests that Si resonators will have a small dynamic range.
In contrast, nanoscale mechanical structures hold the potential to enable the fabrication of high-quality-factor (Q) mechanical resonators with high mechanical responsivity over a wide dynamic range. Such devices can form very low-loss, low-phase-noise oscillators for filters, local oscillators, and other signal processing applications. High-Q resonators are critical components in communications and radar systems, as well as in MEMS-based sensors such as a micro-gyroscope. The combination of high-Q with small force constants enabled by nanoscale resonators would also produce oscillators with exceptional force sensitivity. This sensitivity is important for a variety of force-detection-based sensors and may ultimately allow single molecule spectroscopy by NMR and optical techniques. (M. L. Roukes, Solid-State Sensor and Actuator Workshop Proceedings, Hilton Head, S.C., Jun. 4-8, 2000, p. 367.) Such mechanical oscillators are also key components for mechanical signal processing, which is of great interest because small-scale, high-Q mechanical elements may theoretically enable processing at GHz rates with orders-of-magnitude lower power dissipation than conventional CMOS processors.
Despite the potential for these nanomechanical devices, the practical application of nanotube-based actuators and oscillators has been limited by the development of growth and processing methods for the control of nanotube placement and orientation. These techniques are critical for a wide variety of other nanotube applications including nanotube electronic systems.
One novel approach to making nanometer-scale structures utilizes self-assembly of atoms and molecules to build up functional structures. In self-assembled processing, atom positions are determined by fundamental physical constraints such as bond lengths and angles, as well as atom-to-atom interactions with other atoms in the vicinity of the site being occupied. Essentially, self-assembly uses the principles of synthetic chemistry and biology to grow complex structures from a set of basic feedstocks. Utilizing such techniques molecular motors have been synthetically produced containing fewer than 80 atoms. Chemical vapor deposition (CVD) appears to be the most suitable method for nanotube production for sensor and electronic applications. CVD uses a carbon-containing gas such as methane, which is decomposed at a hot substrate surface (typically 600-900C) coated with a thin catalyst film such as Ni or Co. However, most studies to date have produced disordered nanotube films.
A notable exception is the work of Prof. Xu of Brown University who has developed a new technique for producing geometrically regular nanotube arrays with excellent uniformity in nanopore templates. Xu et al. Appl. Phys. Lett., 75, 367 (1999), incorporated herein by reference. Post-patterning of these ordered arrays could be used to selectively remove tubes in certain areas or produce regions with different length tubes. A variety of other studies have shown that dense, but locally disordered arrays of normally-oriented nanotubes can be selectively grown on pre-patterned catalyst layers.
In addition, there has been little progress in the control of nanotube orientation in the plane parallel to the substrate surface. Many of the basic electrical measurements of nanotubes have been done using electrodes placed on randomly scattered tubes after growth, or by physically manipulating tubes into place with an atomic force microscope (AFM). Dai and co-workers have been able to demonstrate random in-plane growth between closely spaced catalyst pads, including growth over trenches, as well as a related technique to produce nanotubes suspended between Si posts. Dai. Et al., Science, 283, 512 (1999), incorporated herein by reference. In these cases individual nanotubes sometimes contact adjacent electrodes by chance, and excess tubes can be removed with an AFM tip. This type of procedure can be effective for simple electrical measurements, but considerable improvements will be required for production of more complex nanotube circuits. Smalley""s group has demonstrated a wet chemistry-based method of control over nanotube placement using solution deposition on chemically functionalized substrates, although questions remain about nanotube length control and contact resistance. Smalley et al., Nature, 391, 59 (1998), incorporated herein by reference. More recently, Han and co-workers have demonstrated a process for lateral growth of nanotubes between two electrodes, but with uncontrolled nanotube location and orientation in the plane of the substrate (Han et al., Journal of Applied Physics 90, 5731 (2001)). None of the current techniques have been able to grow vertical individual nanotubes or small groups of nanotubes in controlled locations with integrated electrodes, as would be necessary to form nanotube oscillators or actuators.
In addition to the problems associated with controlled growth and orientation of nanotubes, nanotube actuators and oscillators also require a transduction mechanism to convert input signals to physical motion and to provide corresponding output signals.
One possible electromechanical transduction mechanism is suggested by a recent demonstration that nanotube mats can serve as very high efficiency electromechanical actuators in an electrolyte solution, with the possibility of even better results for well-ordered single wall tubes. Baughman et al., Science, 284 1340 (1999), incorporated herein by reference. In brief, Baughman found that electronic charge injection into nanotubes results in a change in the length of the nanotubes. Conversely, it is expected that changing the length of a nanotube through an externally-applied force will result in the movement of charge on or off the tube, depending on whether the tube is stretched or compressed. However, this technique has only been demonstrated for large disordered arrays of nanotubes, no technique has been developed for the controlled motion of individual nanotube resonators. Other potential actuation mechanisms include light-induced nanotube motion, which has been observed, and magnetomotive actuation (M. L. Roukes, Solid-State Sensor and Actuator Workshop Proceedings, Hilton Head, S.C., Jun. 4-8, 2000, p. 367.). However, it is expected that light coupling to nanoscale-cross-section nanotubes will be inefficient, and magnetomotive actuation requires extremely high magnetic fields.
Finally, there has been one report of high frequency resonator measurements on carbon nanotube bundles suspended over a trench in which Qs of around 1000 at 2 GHz were observed. However, the carbon bundle resonator required superconducting electrodes and low temperature measurement.
There have also been a number of papers on using mechanical resonators as mass detectors via measurement of mass-induced resonant frequency shifts (xe2x80x9cMechanical resonant immunospecific biological detectorxe2x80x9d, B. Ilic, D. Czaplewski, H. G. Craighead, P. Neuzil, C. Campagnolo, and C. Batt, Appl. Phys. Lett. 77, 450 (2000); K. L. Ekinci, X. M. H. Huang, and M. L. Roukes, xe2x80x9cUltrasensitive Nanoelectromechanical Mass Detectionxe2x80x9d, preprint Jun. 6, 2001 submitted to Science). Roukes and co-workers even predict an ultimate mass sensitivity with nanomechanical resonators approaching a single Dalton, the mass of a hydrogen atom. However, to our knowledge, there have been no demonstrations of single molecule sensing using mechanical resonators.
Accordingly, a need exists to develop nanoscale mechanical devices, such as, resonators to enable real-world applications ranging from molecular-scale characterization to ultra-low-loss mechanical filters and local oscillators for communications and radar, to rad-hard low-power mechanical signal processors.
The present invention is directed to a tunable nanomechanical resonator system comprising at least one suspended nanofeature, such as a nanotube, where the nanofeature is in signal communication with means for inducing a difference in charge density in the nanofeature (e.g. by applying a capacitively-coupled voltage bias) such that the resonant frequency of the nanofeature can be tuned by changing the tension across the nanofeature, and where the nanofeature is in signal communication with an RF bias such that resonant movement can be induced in the suspended portion of the nanofeature, forming a nanoscale resonator, as well as a force sensor when operated in an inverse mode. The invention is also directed to growth techniques capable of producing a suspended nanofeature structure controllably positioned and oriented with integrated electrodes.
In one embodiment, this invention utilizes a suspended nanotube with integrated electrodes on a substrate that functions as an electromechanical resonator. This invention is also directed to a device, which utilizes a suspended nanotube with integrated electrodes on a substrate that functions as a molecular sensor or a frequency spectrum analyzer. This invention is also directed to novel systems and methods for utilizing devices comprising at least one suspended nanotube with integrated electrodes on a substrate.
In another alternative embodiment, the mechanical oscillation is produced by optical irradiation or capacative coupling.
In still another embodiment, the invention is directed to suspended nanotube oscillators or resonators. The suspended nanotube oscillators may be utilized as high-Q mechanical resonators for filters, signal processing, and sensors. In such an embodiment, excitation and readout of a nanotube oscillator may be made using any suitable methods, including: charge injection, light, or electrostatic.
In still yet another embodiment, the invention is directed to a system for the detection of substances or signal frequencies comprising multiple detectors as described above, such that parallel processing of molecules or signals can be carried out.
In still yet another embodiment, the invention is directed to growth and processing techniques to control resonator location and orientation; and methods for positioning nanotubes during growth, including nanoscale patterning of the substrate to ensure that the growth of the nanotubes is located and aligned with the integrated electrodes.
In still yet another embodiment the nanotubes comprising the resonators are self-assembled into resonators having a specified diameter and height suitable for use in the devices of the current invention.
In still yet another embodiment, the substrate is made of a semiconductor such as, for example, oxidized silicon or aluminum oxide, coated with a metal catalyst film such as, for example, Ni or Co. In this embodiment, the silicon can be further doped to adjust the electronic properties of the substrate surface.
In still yet another embodiment, the nanotubes comprising the resonators are self-assembled from an inert material such as, for example, carbon utilizing a carbon feedstock gas such as, for example, ethylene.